The invention relates to a method and an arrangement for controlling the operation of a high-frequency power amplifier.
In GSM transmitters, for instance, specifications place rigid restrictions also on the operation of the transmitter as well as on switching on or off the transmission power. When the transmission power is switched off at a GSM transmission, it should be possible to reduce the transmission power during idle time slots between frames by at least 70 dB, in practice by 80 dB, which can be realized by decreased gain and by using RF switching means providing an attenuation of about 40 dB, in addition to the 40 dB obtained by the control of the amplifier. One problem with the previous solutions is that the dynamic range of detector means is only about 40 dB.
In some methods and arrangements according to the prior art, the transmission power is switched off by using a switching means positioned in a power control loop on the input side of an amplifier, whereby the signal path breaks, however, and the detector output remains floating in a way. This generates a control signal undefined in its details in a control means of the input side of the amplifier, which in turn causes problems, i.e. practically a control error in a situation when the transmission power shall be switched on again. A control error may also cause a decrease in attenuation when the power is switched off. This restricts the permitted maximum gain between the detector and the control means as well, which maximum gain determines in turn the accuracy of the output power in an ON state.
In another known embodiment, a switching means is positioned in an antenna line outside a power control loop, which requires, however, an amplifier and an RF switch with a higher power, which leads to bigger power losses.
It is also known from the prior art to use a logarithmic RF range amplifier and detector to expand the dynamic range of the detector, but the component in question is an expensive hybrid component and thus not necessarily a suitable choice always.
From European Patent Application 0 135 154 is known an apparatus for controlling transmission power, but no reference is made therein to bringing an amplifier to an OFF state. The objective of the apparatus disclosed in that publication is to minimize variations in loop gain caused by the non-linearity of the power amplifier. The EP publication relates to controlling DC gain and by no means to controlling RF gain or RF attenuation, which is the case in the present invention. In the solution disclosed in the EP publication, the control of a power control loop comes from inside the loop.